156 EMBRYOGENESIS IN PLANTS 



lower or inner portion of the megaspore is filled with food reserves. 

 On further grov/th the prothallus breaks through the spore membrane 

 and some chlorophyll develops. If the first archegonium in Azolla is 

 fertilised, no other archegonium is differentiated; otherwise several 

 others may be formed. In Salvinia the prothallus is large, with more 

 green tissue and more archegonia. The initial development of the 

 archegonium is as in the homosporous ferns, but at maturity the neck 

 is shorter. 



In both genera the megaspores float with the prothallus and arche- 

 gonia directed upwards. In Azolla filiculoides the zygote elongates 

 vertically and its first division is usually by a transverse wall, i.e. 

 the basal wall is at right-angles to the neck of the archegonium. 

 Fig. 36g, h. More oblique divisions, however, have also been illustrated. 

 Fig. 36f. The next wall is at right angles to the first, the two epibasal 

 quadrants giving rise to the shoot apex and first leaf, and the two hypo- 

 basal ones to the foot and root. Fig. 36h, j. A 'two-sided' apical cell is 

 formed by the first division of the stem quadrant ; this cell persists in 

 the adult sporophyte. The leaf quadrant divides by a median wall, and 

 this, according to Campbell (1918), foreshadows the two lobes found 

 in older leaves. The first leaf, in fact, becomes a funnel-shaped sheath 

 within which lies the shoot apex. The first root is formed as in the 

 Polypodiaceae. The foot is a large organ and becomes elongated below 

 the root-base. The second leaf originates from the first segment of the 

 shoot apical cell, each successive segment forming a new leaf. Vascular 

 tissue can apparently not be distinguished until the second leaf has 

 grown considerably, the strands from the first and second leaves and 

 from the primary root being conjoined near the centre of the shoot, 

 Fig. 36j. No vascular tissue can be detected below the shoot meristem. 

 The embryogeny of Salvinia resembles that of Azolla but no roots are 

 formed. According to Yasui (1911) a root rudiment is present in the 

 early embryogeny but it does not develop further and becomes merged 

 with the tissues of the foot. 



Although the segmentation pattern and organogenic developments 

 in Azolla and Salvinia are closely comparable with those in the Poly- 

 podiaceae, we may note that it is the two upper segments, next to the 

 archegonial neck, which give rise to the shoot apex and first leaf 

 respectively : in leptosporangiate ferns, the corresponding two segments 

 give rise to the first leaf and first root respectively. These facts suggest 

 that gravity may be one of the factors which affects the biochemical 

 pattern underlying the embryonic development. 



Parthenogenetically formed embryos of Marsilea dnmvnondii are 

 strictly comparable with the normal embryo {see next Section). 



